It is desirable to allow users of a “home” LAN to access that LAN even when they are away from the home LAN environment. In many cases, this means allowing the user to connect to a LAN in his current location, e.g. a WLAN hotspot or a public Ethernet access, and through that connection to the home LAN.
The inter-connection of private, e.g. house and office, networks across intervening broadband networks is possible using already standardised solutions (or solutions under standardisation). For example, work carried out by the Internet Engineering Task Force (IETF) deals with providing so-called “pseudo wires” between access points to an IP or Multiprotocol Label Switching (MPLS) network and which emulate Ethernet services over the IP or MPLS network. Work carried out by the IEEE deals with LAN interconnection solutions, where the intermediate operator-provided network is also itself an Ethernet network.
Ethernet is an example of a Layer 2 (L2) networking protocol. In order to enable the inter-connection of different Ethernet networks, L2 tunnels and L2VPN solutions are required. One such solution is that known as “EtherIP” [IETF RFC3378], which encapsulates Ethernet packet at a sending side LAN with an IP header, and sends the packets to an access router (AR) of the receiving side LAN which decapsulates the packets and bridges the Ethernet frame as if it was a L2 node (switch, bridge). Another currently standardised solution is to use L2TPv3 [RFC4719, “Transport of Ethernet Frames over Layer 2 Tunnelling Protocol Version 3 (L2TPv3)”] to transport Ethernet frames in an IP/UDP packet. An extension of L2TP for L2VPN usage also exists and which enables L2 tunnels to be connected to virtual bridges, so that standard Ethernet MAC forwarding can be used along with MAC learning to forward the Ethernet frames received by the virtual switch [RFC4667, “Layer 2 virtual private network (L2VPN) extensions for layer 2 tunneling protocol (L2TP)”].
Different solutions exist for LAN inter-connect over MPLS provider networks. One solution is a pseudo-wire over MPLS as described in RFC4448, “Encapsulation Methods for Transport of Ethernet over MPLS Networks”. Also, The L2VPN working group of the IETF is standardising a Virtual Private LAN Service (VPLS) to transport L2 protocol data units over IP/MPLS networks [IETF WG Layer 2 Virtual Private Networks (l2vpn), http://ietforg/html.charters/l2vpn-charter.html]. VPLS is also referred to as Transparent LAN Service and Virtual Private Switched Network service. It incorporates MAC address learning, flooding, and forwarding functions in the context of pseudo-wires that connect individual LANs across a packet switched network. Implementations of VPLS in an IP/MPLS router often refer to the virtual bridge functionality as VSI (Virtual Switch Instance).
There exist solutions to inter-connect two (or more) LANs using an intervening L2 operator (provider) networks. If the provider's network is Ethernet, then the 802.1ad Q-in-Q method or the 802.1ah Mac-in-Mac method could be used. In the case of 802.1ad, the original 802.1Q header has been extended with an additional virtual LAN (VLAN) tag called “S-VLAN” (Service VLAN), which makes it possible to build up a virtual network structure. Whilst the original VLAN tag defined in the 802.1Q standard is transmitted transparently by 802.1ad capable switches, a virtual topology can be defined between 802.1ad capable switches by configuring VLANs appropriately on the switch interfaces, and switching and forwarding based on the S-VLAN tag. The switches on the border of an 802.1ad and 802.1Q segment are responsible for generating and stripping the S-VLAN tag.
One possible application of 802.1ad is to use the S-VLAN tag for identification of a service. It is important to note that, in the case of 802.1ad, the customer and provider network are not separate, so both networks appear as a large domain, causing for example an STP action in a customer network to impact on the provider network.
802.1ah MAC-in-MAC provides for the encapsulation of an entire Ethernet frame and a new MAC header is generated by the edge switch on the border of an 802.1ah domain. Within the domain the original Ethernet frame is transmitted transparently, with all switching and forwarding decisions in the intermediate switches being based upon the destination MAC address and VLAN tag(s) in the outer MAC header. The main benefit of 802.1ah is its scalability to large Ethernet networks, and in addition the clear separation that it provides between the provider and customer Ethernet networks.
Existing L2VPN solutions rely on static configuration and assume fixed locations for customer sites. Mobile users are therefore restricted to using only pre-configured customer sites. On the other hand, while L3 mobility solutions such as MobileIP (MIP) potentially offer unrestricted access, they do not offer L2 transparency. This means that if a terminal uses L2 specific protocols (e.g. protocols relying on broadcast L2 frames), the L3 solution will not work. Moreover, L3 mobility solutions do not allow overlapping addresses for mobile hosts. Although a terminal may reside in a remote subnet with its home IP address, this home IP address must be unique. Thus for example two mobile terminals cannot have the same home IP address in the same access network. Unfortunately, home users often use private address spaces which may very easily be overlapping.
Some L3 mobility solutions require the implementation of dedicated protocols in terminals, and which are often not available (e.g., MIP is not available in Windows™). L3 solutions are to some extent dependent on the version of IP. However, if mobility is transparent at layer 2, then the terminal may use either IPv4 or IPv6 in its home LAN.